Every selective call network system has an associated coverage area corresponding to the reception area of the system's transmitters which relay messages at an assigned frequency from the selective call network terminal to selective call receivers. As a selective call receiver increases its distance from the network transmitters, reception on the assigned frequency becomes more difficult until, eventually, the selective call receiver is out of range of the transmitters.
It is desirable that a selective call receiver user be aware of when the selective call receiver is out of range. The receiver will not receive any messages while out of range. If the user is expecting an important message, detection of out of range will indicate that the user needs to return to the network coverage area or access alternative methods for retrieving messages.
Selective call network signalling typically utilizes one or two binary frequency shift keying (FSK) modulation signalling protocols: either the POCSAG signalling code or the Golay Sequencing Code (GSC), or both. POCSAG is a protocol originally proposed by British Telecom and an anagram for the Post Office Code Standardization Advisory Group. The POCSAG protocol receivers perform separate bit and frame synchronization operations. Bit synchronization is a process used to determine the presence of bit boundaries of a data transmission having bits transmitted at a predetermined baud rate, and thereafter to provide a clock to synchronously sample the bits. Frame synchronization frames the bits transmitted so that the data transmitted is in a form to be decoded thereby indicating various word boundaries such as the first bit of address and data signals.
The preamble of a POCSAG signal comprises a number of zero-to-one transitions. One method for POCSAG bit synchronization is described in U.S. Pat. No. 4,506,262. Generally, with POCSAG signalling, once bit synchronization has been obtained, the receiver remains in synchronous communication with the transmitter until the signal is dropped, i.e., bit synchronization does not need to be processed repeatedly. The first word following the preamble is a sync code word which contains a predetermined binary sequence used for frame synchronization. Sixteen words of information follow before the sync code is repeated. The sync code provides a means for detection of frame synchronization. It can be appreciated that if the POCSAG protocol is ignored, the data bits can appear to be effectively a random sequence of data transmitted at a predetermined baud rate, normally 512 bits per second (bps).
Since frame synchronization is repeated every seventeenth word of transmission, POCSAG selective call receivers have an inherent means of out of range detection. If frame synchronization has been detected, the POCSAG selective call receiver looks in the following information for its address and an associated message. If no frame synchronization has been detected within a period of time substantially greater than the time to transmit sixteen words (five minutes, for example), the POCSAG selective call receiver can declare out of range.
With selective call receivers using the Golay Sequencing Code (GSC), out of range detection cannot be detected using frame synchronization. Unlike POCSAG, GSC selective call receivers must simultaneously gain bit and frame synchronization. The preamble of the GSC signal, lasting more than one second, contains a repeated twenty-three bit predetermined sequence of signals. In a GSC paging network, the receivers are divided into a plurality of groups and the twenty three bit code in the preamble selects only a first one of the groups. Synchronization to the GSC signal is disclosed in U.S. Pat. No. 4,424,514. Thus other groups of GSC selective call receivers do not synchronize to a GSC signal which selects the first group. Furthermore, since GSC is transmitted at 600 bps, POCSAG selective call receivers cannot bit synchronize to the GSC signal even though the POCSAG receivers are in range. Therefore, the GSC receivers must sample the transmission until it detects synchronization with its own predetermined preamble. Following the preamble, a start code indicates that the preamble is finished and the information following contains addresses of selective call receivers within the group and information in a pattern of eight information words followed by one start code word. The GSC receivers within the network coverage area which are not in the group preambled, do not detect synchronization and therefore cannot detect whether they are out of range of the transmitter or whether their group has not been transmitted to by the transmitter. Furthermore, because the POCSAG signal does not contain the 600 bps twenty-three bit preamble signal required for a single group of GSC selective call receivers to synchronize, GSC selective call receivers cannot detect a POCSAG signal even though they are in range of the transmitter.
Thus, what is needed is a method for detecting if a selective call receiver is out of range of its transmitter for both the GSC and the POCSAG signalling protocols.